Re: irq-disabled vs vmap vs text_poke

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2009-02-13 at 13:55 +0100, Nick Piggin wrote:
> On Fri, Feb 13, 2009 at 01:50:07PM +0100, Peter Zijlstra wrote:
> > Hi,
> > 
> > Ingo got the following splat:

<snip splat>

> > Which points to vunmap() being called with interrupts disabled.
> > 
> > Which made me look at the vmap/vunmap calls, and they appear to not be
> > irq-safe, therefore this would be a bug in text_poke().
> > 
> > [ that is, vmap() can end up calling get_vm_area_caller() which in turn
> >   calls __get_vm_area_node() with GFP_KERNEL, ergo, don't do this from
> >   an atomic context. ]
> > 
> > Now text_poke() uses local_irq_save/restore(), which conveys that it can
> > be called with IRQs disabled, which is exactly what happens in the trace
> > above, however we just established that vmap/vunmap() are not irq-safe.
> > 
> > Anybody got an idea on how to fix this?
> 
> Oh, I thought the consensus was not to use vmap for this?

Seems like a sensible consensus, still that means text_poke() needs some
TLC.

> With a bit of work, we can make vunmap irq-safe with the lazy vunmapping
> infrastructure (vmap could also be irq-safe, but would be subject to
> spurious failures due to being unable to flush lazy vunmaps.

*nod*

> I think I got a mostly working patch cobbled together sitting here
> somewhere. I was waiting for some _really_ good use case before spending
> more time on it. I would prefer if at all possible to do vmap operations
> in sleepable, process context.

Agreed, I think we want to fix text_poke() and make the vmap/vunmap()
ops yell louder at violations of these rules.

I'm just totally clueless wrt text_poke() hence this email ;-)